The “port identifier” field is one component of the spanning tree priority vectors that are used as the basis of Spanning Tree Protocol's computation of an active topology. According to existing implementations of Spanning Tree Protocol (STP), the port identifier is encoded as two octets (i.e., bytes), which are taken to represent an unsigned binary number. For example, FIG. 1 depicts the format of a conventional configuration bridge protocol data unit (BPDU) 100 that is used by STP according to an embodiment. As shown, BPDU 100 encodes the port identifier at octets 26 and 27.
When two port identifiers are compared by a network device (e.g., bridge) that receives BPDUs such as BPDU 100 of FIG. 1, the lesser number generally denotes the port of higher (i.e., better) priority. The more significant octet of the port identifier is a settable priority component that allows the relative priorities of ports on the same bridge to be managed. The less significant 12 bits correspond to the port number, expressed as an unsigned binary number. Valid port numbers are in the range 1 through 4095. Typically, the value 0 is not used as a port number.
The most significant 4 bits of the port identifier correspond to the priority for the port, which may be set to manage the active topology of the network. The table below specifies default values and ranges for port identifier priorities. If this priority parameter can be updated by management, the bridge can have the capability to use the full range of values with the granularity specified.
TABLE 1Recommended or default valueRange1280-240 in steps of 16
It should be noted that the number of bits that are considered to be part of the port number (least significant 12 bits) differs in the most current and prior versions of the STP standard. In prior versions, the priority component was 8 bits and the port number component was also 8 bits. This was changed to 4 bits and 12 bits respectively in the most recent version. This change acknowledged that modern switched LAN infrastructures call for increasingly larger numbers of ports to be supported in a single bridge. To maintain management compatibility with older implementations, the priority component in the current STP standard is still considered, for management purposes, to be an 8-bit value, but the values that it can be set to are restricted to those where the least significant 4 bits are zero (i.e., only the most significant 4 bits are settable, hence resulting in steps of 16).
In modern systems of devices (e.g., systems of network devices), the number of devices that are supported has increased drastically due to Institute of Electrical and Electronics Engineers (IEEE) 802.1BR bridge port extension support. For example, some systems of devices can support up to 12 controller bridge (CB) units and 40 port extender (PE) units, with each unit having 128 logical ports. Thus, in these systems of devices, the number of ports that can, or need to, run STP is 6656 (52*128). Further, in the future, the number of units that are supported may climb even higher.
However, as noted above, the current STP standard is only capable of supporting 4095 unique ports (identified by a 12 bit port number component) due to the format of the port identifier field. Accordingly, supporting STP in a modern system of devices with more than 4095 ports is not currently possible.